Early in the afternoon of Sunday July 25th, 2010, a squall
line of thunderstorms formed over eastern West Virginia. With the heat
of a near 100 degree afternoon to fuel it, and winds that strengthened to
70 mph higher up in the atmosphere where these clouds were billowing,
that
line
intensified as it moved rapidly east.

It
would end up being a prolific wind storm for the area, with the northern
suburbs and northern D.C. particularly hard hit. Winds gusted up to 90 mph
in a few
spots that were hardest hit, and there were several measured wind gusts between
60 and 70 mph in areas that received damage. With that destructive wind,
hundreds of trees were felled along the entire path of the squall line, power
was knocked
out
to hundreds
of thousands, and many roads were blocked. Trees fell onto houses and vehicles.
There were some reports of minor roof damage, as was the case in suburban
Prince George County at 2 different apartment complexes. The falling trees
also killed two: one boy in Loudoun County VA who was walking through
a park
with
his
family,
and a woman in College Park MD whose van was stopped in its tracks when a
tree fell on it. In addition, there was a lightning fatality reported in
Rockville, MD when a man was struck outdoors while attending a community
celebration. The storm also caused or contributed to the death of a man who
was jet skiing on the Chesapeake Bay, a half mile south of the Bay Bridge
when lightning struck nearby.

A squall line begins as
individual thunderstorms that form along a boundary in the atmosphere.
If winds are aligned in a certain way through the atmosphere, these storms
will begin to organize and coalesce into a line
of
thunderstorms - a squall line.
This typically happens several times during the course of a mid-Atlantic
summer as cold fronts push though warm and humid summertime air.

So why
was this one so strong and damaging? In this case, everything was pushed
to the extreme. Temperatures rose to near 100 degrees (near record level
for the day) with high humidity. That hot, humid
air is the fuel for thunderstorms, and it was in abundance. That fuel was
combined with winds in the atmosphere that steadily increased during the
morning
and into the afternoon. While winds were only gusting 10 to 20 mph at
ground level, by early afternoon winds around 15,000 feet (the middle of
the thunderstorms) had increased to 70 mph. Those strong winds higher
up
in the atmosphere were tapped in the turbulence of the thunderstorms,
and thrown down to the surface in damaging wind gusts.

Radar

Below is a time lapse
of squall line at 2:30pm, 3:00pm, 3:30pm, 4:00pm. Imagery is from the NWS
Doppler Radar located here at the Baltimore/Washington forecast office
in Sterling, VA.

While the standard radar pictures above showing the heavy
rain does not look too far out of the ordinary for a summertime line

of
thunderstorms, the NWS radar can look into these storms and see the damaging
winds that are
associated with them. These next images are from the same four times as above,
but instead show the winds within the storms. Arrows point to areas where the
radar indicates winds higher than 50 mph, and in some spots more than
70 mph.

Reported Damage and Impacts

From those damaging winds, trees were downed along the squall
line's path. Here is a map of damage reported to the National Weather Service.
On this map, every
'W'
is
a report
of wind
damage.
The blue
areas
are areas
that were issued Severe Thunderstorm Warnings by the National Weather Service.

This is a map of trees damage just in the District of Columbia
alone (dots):

In the main damage area shown in red here...

...winds gusted 40 to 50 mph on a widespread basis. That was
enough to tear leaves and small branches off trees over that entire area, and
even knock down some weaker trees. Within that area however, there were spots
that received winds of 50-70 mph which was enough to bring down trees and large
branches. Some of those spots that had a wind instrument to measure the wind
speed:

And
then there were a few isolated spots where this squall line threw down
its most extreme wind gusts of 80-90mph. In those spots, large trees
snapped, even power poles were pushed over or snapped, not from the force
of a tree falling on them, but from wind force alone.

Danger Brewing (Outlook
-> Watch
-> Warning)

NWS meteorologists were monitoring the developing dangerous
weather conditions during the late week, and by Friday morning began to include
that threat in the Hazardous Weather Outlook with thunderstorms that "may
produce strong gusty winds." That threat continued in the outlook through the
weekend. As NWS meteorologists became even more confident on during the morning
and midday hours on Sunday, a Severe Thunderstorm Watch was first issued for
an areas including northern
MD, northwestern
VA,
and northeast WV. That was followed by another Severe Thunderstorm Watch for
an area just south of the initial one, including Washington D.C.

As the squall line moved rapidly across the region,
Severe Thunderstorm Warnings were to be issued. A Severe Thunderstorm
Warning was issued for the area around Winchester, Front Royal, Berryville,
and Charles Town at 2:01pm.

That was followed by a warning for the outer northwest
D.C. suburbs at 2:36pm.

And then one for Washington and central MD at 3:07pm.

(in the above maps, red 'pins' are reported damage locations)

Other warnings were issued as well. All in all that Sunday,
eight Severe Thunderstorm Warnings were issued, 7 Special Marine Warnings for
the Chesapeake Bay and its major estuaries, and one Tornado Warning for Howard
and Montgomery Counties. As of the following Wednesday morning, NWS Storm Survey
teams dispatched for that warning area could find no conclusive evidence of
a tornado, only strong damaging winds associated with the squall line.

Always remember the NWS uses the watch/warning program to
alert you to ANY weather threat! The watch/warning system is much like a traffic
light. Green is OK. If there is a possible threat, the NWS will issue a watch
saying there is the potential for that threat to happen, so KEEP A WATCH OUT
(a yellow
light, caution)! If we move up to a WARNING (a red light), that means we expect
something to occur, so take action!

In this case, a Severe Thunderstorm Watch
means we have the potential for severe thunderstorms, so be ready and KEEP
A WATCH OUT! A Severe Thunderstorm Warning means we expect a severe, damaging
thunderstorm
to occur in a targeted area, within a short period of time (generally 30-60
minutes)...so TAKE ACTION AND SEEK SHELTER!

For future events, if you would like to better receive
warnings from the National Weather Service, you can do any or all of the
following: